1. Field of the Invention
The present invention relates to an improvement of a driving apparatus for driving a brushless motor, and more particularly to the driving apparatus for driving the brushless motor that provides a stable rotational speed by a smooth braking.
2. Description of the Related Art
Recently, a brushless motor is becoming increasingly common in place of a DC motor with a brush. The brushless motor comprises a commutation system that electromagnetically detects a rotation of a rotor and switches application of a current to magnetic coils, instead of a mechanical commutation system that comprises a mechanical brush and a commutator. Thus, the brushless motor, compared with the motor provided with the mechanical commutation system, has advantages of a reduction of a noise resulting from the rotation and a high durability.
The brushless motor is employed as a driving source for various kinds of apparatus such as conveyor lines.
In the conveyor lines incorporating the brushless motor, not only a flat conveyor line but also an inclined line is formed. However, in the inclined conveyor line, specifically in a line having a descent inclination, a conveying speed of an article is increased by a force of gravity at the descent inclination part, resulting in increasing the brushless motor's rotational speed along with the increase of the speed of the articles. It has caused problems such as an increase of an impact when making the accelerated articles to stop, and/or an increase of an inertia resulting from the increase of the conveying speed, thereby making a stop position unstable.
In order to prevent the brushless motor from the increase of the rotational speed resulting from such an external force on the motor, several means for a braking control so as to reduce the rotational speed are previously suggested. For example, a patent document specified below discloses a construction that effects a braking by reversing a rotational direction with maintaining application of a current to the brushless motor.
Patent Document: Japanese Patent Laying-Open Gazette No 06-276785.
However, the braking control disclosed in the patent document requires application of a large excitation current so as to reverse the rotational direction, and causes a remarkable increase of the electrical current. Further, on starting the braking control, an abrupt braking force acts against the rotation at that time, resulting in a vibration and an impact, which generates noise, so that the improvement has been expected.